Write-back is known as a data write method using a cache. For example, when controlling a write into a memory device by the write-back method, a control apparatus replies to a host when write data transmitted from the host is written into a cache. Then, the control apparatus writes the write data of the cache into the memory device asynchronously from the write into the cache, within an idle time during subsequent processes for example.
Here, as one example of technology relevant to cache control, the next technology is proposed. In this technology, a battery monitoring circuit monitors a battery for supplying electric power to a cache memory at the time of power outage. The disk control circuit controls a disk to perform a write-back operation when the battery monitoring circuit determines that the battery is in a normal state, and controls the disk to perform a write-through operation when the battery monitoring circuit determines that the battery is in an abnormal state.
Also, in a proposed storage system, a plurality of control apparatuses include respective local caches and mirror caches, and the local cache of a certain control apparatus is duplicated in the mirror cache of an adjacent control apparatus, so that the caches are duplicated cyclically.
See, for example, Japanese Laid-open Utility Model Publication No. 63-5539 and International Publication Pamphlet No. WO 2004/114115.
In the meantime, as an example of a storage system including a plurality of control apparatuses, one can conceive of a system including control apparatuses each having a cache battery. For example, one can conceive of a system including control apparatuses provided in different housings. In this system, an abnormal state of the battery of a certain control apparatus increases the risk of losing data of a cache of the control apparatus, and thus the control apparatus performs an operation for protecting the data of the cache.
However, when the caches are cyclically duplicated as described above in this system, an abnormal state of the battery of a certain control apparatus increases not only the risk of data loss of the local cache in the control apparatus, but also the risk of data loss of the mirror cache in the control apparatus. This compromises safety of not only the data in the control apparatus, but also data of a local cache in another control apparatus corresponding to the mirror cache. That is, the cyclically duplicated cache configuration has a problem of compromising the safety of data in another control apparatus other than the control apparatus including the battery in an abnormal state.